1. Field of the Invention
The present invention generally relates to power supply units and, more particularly, to a power supply unit for an appliance carrying electric batteries.
Appliances such as a notebook personal computer carrying an electric battery are usually operable using an AC adaptor for converting a commercial AC power into a DC power or the electric battery carried in the appliance. Also, such an appliance is usually equipped with a charger so that the electric battery is charged while the AC adaptor is being used.
2. Description of the Related Art
FIG. 1 shows a construction of a conventional power supply unit.
Referring to FIG. 1, the power supply unit comprises an AC adaptor 111, an electric battery 112, a charger 113, a current sense circuit 114, an D/D converter 115, an A/D converter 117, a microprocessor 118, a discharge control circuit 119 and butt diodes D1-D3.
The AC adaptor 111 provides, under normal conditions, a higher voltage than the electric battery 112 implemented by, for example, a lithium battery.
The charger 113 charges the electric battery 112 with power supplied by the AC adaptor 111.
The current sense circuit 114 detects a charge current and a discharge current of the electric battery 112.
The D/D converter 115 converts the output voltage of the AC adaptor 111 or the electric battery 112 to a voltage adapted for a load 116.
The A/D converter 117 converts an analog current sensed by the current sense circuit 114 into a digital value.
The microprocessor 118 controls the charge level and the discharge level of the electric battery 112.
The discharge control circuit 119 controls the discharge level of the electric battery 112.
The butt diode D1 prevents the power of the electric battery 112 from being drained to the AC adaptor 111 while the electric battery 112 is in use.
The butt diode D2 prevents the power of the electric battery 112 from being drained to the charger 113 while the electric battery 112 is in use.
The butt diode D3 prevents the power of the AC adaptor 111 from being drained to the current sense circuit 114.
A description will now be given of the operation according to the construction in FIG. 1.
When the AC adaptor is in use, the power thereof is fed to the D/D converter 115 via the diode D1. The D/D converter generates a voltage adapted for the load 116 from the power supplied thereto. As described above, the diode D3 prevents the power supplied by the AC adaptor 111 from being drained to current sense circuit 114.
The power of the AC adaptor is also supplied to the charger 113 which is in control of the charging. The charge current is fed to the input of the current sense circuit 114 via the diode D2. The charge power is transmitted from the current sense circuit 114 to the electric battery 112. An analog current sensed by the current sense circuit 114 is fed to the A/D converter 117. The A/D converter 117 converts the analog current into a digital value and notifies the MPU 118 of the level of the digital current. The MPU 118 calculates the charge level based on the current output from the A/D converter 117. Upon detecting an overcharge, the MPU 118 notifies the charger 113 of the overcharge.
A description will now be given of an operation in which the power is supplied from the electric battery 112 to the load 116.
When the AC adaptor 111 fails, the cathode voltage of the D1 and D3 drops to a level lower than the voltage of the electric battery 112, causing the power of the electric battery 112 to be fed to the load 116 (hereinafter, we will refer to a state in which the AC adaptor 111 is disconnected in the power supply unit and a state in which the output voltage thereof drops to an abnormally low level, usually below the voltage of the electric battery, as a power failure).
The power of the electric battery 112 is subject to the discharge control effected by the discharge control circuit 119 and then fed to the D/D converter 15 via the current sense circuit 114 and the diode D3. The D/D converter 115 then generates a voltage adapted for the load 116 from the voltage supplied thereto. As described before, the diode D2 prevents the power supplied from the electric battery 112 from being drained to the charger 113, and the diode D1 prevents the power from the electric battery 112 from being drained to the AC adaptor 111.
The current sense circuit 114 detects the discharge current of the electric battery 112 and supplies the detected analog current level to the A/D converter 117. The A/D converter 117 converts the analog current level into a digital value and transmits the same to the MPU 118. The MPU 118 monitors the discharge current on the basis of the digital value of the discharge current generated by the A/D converter 117. In the event that an overdischarge occurs, the MPU 118 notifies the discharge control circuit 119 accordingly. The discharge control circuit 119 controls the discharge by, for example, stopping the discharge in response to an associated instruction from the MPU 118.
FIG. 2 shows a construction of a conventional charger. FIG. 2 specifically shows a construction for constant-voltage/constant-current charging required of a lithium battery.
Referring to FIG. 2, a charger 113 comprises a flyback circuit formed of a coil L, a resistor R, a capacitor C and a diode D4; a switch circuit 121; a gate-controlled circuit 122; and a charge control signal generation unit 120. The charge control signal generation unit 120 includes a PWM comparator 123, error amplifiers 124 and 125 and an oscillator 126.
The switch circuit 121 turns an input voltage Vin from the AC adaptor ON and OFF so as to control an output voltage Vout of the charger 113.
The gate-controlled circuit 122 controls the ON/OFF action of the switch circuit 121.
The PWM comparator 123 receives the outputs of the error amplifiers 124 and 125, and compares the received outputs with a reference voltage provided by the oscillator 126 so as to generate a control signal to control the operation of the gate-controlled circuit 122.
The error amplifier 124 detects the level of the current output from the charger 113 by receiving the voltage generated across a current sense resistor R.
The amplifier 125 detects the output voltage Vout.
The oscillator 126 generates the reference voltage for the PWM comparator 123.
The flyback circuit formed of the coil L, the resistor R, the capacitor C and the diode D4 produces an oscillating current from the input voltage Vin which is turned ON and OFF by the switch circuit 121. The oscillating current is rectified by the diode D4, producing a DC output voltage.
A description will now be given of the characteristic of the operation of the charger 113.
FIGS. 3A and 3B show relationships between the inputs of the PWM comparator 123 and the output thereof, the inputs of the PWM comparator 123 being the voltages supplied by the error amplifiers 124 and 125, and the oscillator 126.
The oscillator 126 feeds a voltage having a saw-tooth waveform as shown in FIGS. 3A and 3B to the PWM comparator 123. The PWM comparator 123 compares the output voltages of the error amplifiers 124 and 125 with the saw-tooth voltage. While the saw-tooth voltage is higher in level than the higher one of the voltage of the error amplifier 124 and the voltage of the amplifier 125 voltage, the PWM comparator 123 outputs a signal for turning the gate-controlled circuit 122 ON. For example, referring to FIG. 3A, in case the voltage of the amplifier 125 is higher than the voltage of the error amplifier 124, the PWM comparator 123 outputs a signal for turning the gate-controlled circuit 122 ON while the saw-tooth voltage is higher than the voltage of the amplifier 125 (constant-voltage control). Referring to FIG. 3B, in case the voltage of the error amplifier 124 is higher than the voltage of the amplifier 125, the PWM comparator 123 outputs a signal for turning the amplifier 122 ON while the output voltage of the error amplifier 124 is higher than the saw-tooth voltage (constant-current control).
FIG. 3C shows a relationship between the output current and the output voltage of the charger 113. When the output current is lower than a predetermined level Ia, the charger 113 outputs a constant voltage Va (constant-voltage operation). When the output current reaches the predetermined level Ia, the charger 113 outputs the constant current Ia (constant-current operation).
While the charger 113 shown in FIG. 2 is adapted for constant-current/constant-voltage operation, the charger 113 may be adapted for constant-current operation in the case of a NiCd battery, a NiMH battery or the like where constant-current charging is performed. In such a case, the amplifier 125 for detecting the charge voltage is not necessary. The PWM comparator 123 is only required to compare the output of the error amplifier 124 for detecting the charge current with the output of the oscillator 126.
A description will now be given of the operation of the conventional charger shown in FIG. 2.
The input voltage Vin supplied by the AC adaptor is turned ON and OFF by the switch circuit 121. The transient current flows through the coil L, the resistor R, the capacitor C and the diode D4 so as to generate the output voltage Vout. The relationship between the output voltage Vout and the input voltage Vin is EQU Vout=Ton.times.Vin/Ts
Ton: an ON-state interval while the switch in the switch circuit 121 is turned ON. PA1 Ts: an ON-OFF action period of the switch circuit
Accordingly, the longer the ON-state interval of the switch, and the shorter the ON-OFF action period, the higher the output voltage Vout.
As described before, the error amplifier 124 detects the level of the current output from the charger 113 by receiving the voltage generated across the current sense resistor R. The amplifier 125 detects the output voltage Vout by comparing the voltage input thereto with a reference voltage. The PWM comparator 123 compares the outputs of the error amplifiers 124 and 125 with the oscillation voltage of the oscillator 126.
The PWM comparator 123 produces a signal for turning the gate-controlled circuit 122 ON and a signal for turning the gate-controlled circuit 122 OFF in accordance with the condition illustrated in FIGS. 3A and 3B. In response to the signal output from the PWM comparator 123, the gate-controlled circuit 122 turns the switch circuit 121 ON and OFF. In case the voltage output from the amplifier 125 is higher than that of the error amplifier 124, constant-voltage control is performed such that the switching takes place depending only on the output voltage variation of the amplifier 125. In this way, the output voltage of the charger is maintained at the constant level of Va. In case the voltage output from the error amplifier 124 is higher than that of the amplifier 125, constant-current control is performed such that the switching takes place depending only on the output of voltage variation of the error amplifier 124. In this way, the output current of the charger is maintained at the constant level of Ia.
FIG. 4A shows a construction of the conventional current sense circuit. FIG. 4B shows the characteristic of a current flowing in a current sense resistor versus an output voltage of the current sense circuit.
Referring to FIG. 4A, the current sense circuit 114 comprises operational amplifiers 131, 132 and 133; a resistor R' provided for detection of the discharge current or the charge current; resistors r1, r2, r3, r4, r5, r6 and r7; a butt diode D1 coupled to the AC adaptor; and a butt diode D3 connected to the D/D converter 115.
Given that r2=r3, r4=r5 and r6=r7 in the construction of FIG. 4A, the current Is flowing in the resistor R' and the output voltage Voin are related to each other according to the following equation. EQU Voin=(r7/r4)*(1+(2r2/r1))*R'*Is+Vref
Accordingly, the characteristic of the current Is flowing in the resistor R' versus the output voltage Voin is as shown in FIG. 4B. The level and direction of the current Is can be determined by detecting the output voltage Voin, the direction of the current Is being dependent on whether the charge current or the discharge current flows in the current sense resistor R'.
The conventional power supply unit requires that the butt diodes D1, D2 and D3 be coupled to the AC adaptor, the charger, and the D/D converter, respectively. Such an arrangement gives rise to a high cost and a large area required to mount the power supply unit.
The conventional power supply unit also requires two current sense resistors, that is, the current sense resistor R in the charger R and the resistor R' in the current sense circuit, the resistor R detecting the charge current and the resistor R' detecting the charge current and the discharge current. For precise detection of the current level, the resistors R and R' must have a relatively large size, thus increasing the area and cost required for mounting. It is also to be noted that the resistors are best eliminated because they consume power without producing any use.